On/off control for a balanced differential current mode driver

Abstract

A differential current mode driver is provided with output source and sink currents that remain nearly equal in magnitude and opposite in direction throughout normal, power-down, and power-up modes of operation. Three time constants are employed to regulate these different modes of operation. The differential current mode driver includes a first time constant to stabilize the output source and sink currents during the on-state, a second time constant to control the transition from the on-state to tristate, and a third time constant to control the transition from tristate to the on-state, all the while maintaining equal source and sink currents.

What is claimed is: 
     
       1. A driver comprising: single-ended to differential signal means for generating a differential signal from a single-ended input; and   filtering means for filtering the differential signal, wherein the filtering means imposes a first time constant on the differential signal when the driver is in an on-state, a second time constant when the driver transitions from the on-state to a tristate, and a third time constant when the driver transitions from the tristate to the on-state.   
     
     
       2. The driver of claim 1, wherein the single-ended to differential signal means further comprises: control means for controlling the filtering means;   input current means for generating a reference current from a bias current input;   output current means for generating a source current and a sink current from the reference current; and   output switches directing the source current to a first output and the sink current to a second output in response to a first state of the single-ended input signal and directing the source current to the second output and the sink current to the first output in response to a second state of the single-ended input signal.   
     
     
       3. The driver of claim 2, wherein the input current means further comprises: first and second transistors connected in series, wherein a drain and a gate of the first transistor are connected to the bias current input and a source of the second transistor is connected to a power supply voltage;   third and fourth transistors connected in series, wherein a source of the third transistor is connected to a gate and a drain of the fourth transistor, a source of the fourth transistor is connected to the power supply voltage, and the gate of the fourth transistor is connected to a gate of the second transistor;   a fifth transistor having a source connected to a gate of the first transistor, a drain connected to a gate of the third transistor, and a gate connected to a first control signal from the control means; and   a sixth transistor having a source and a drain connected to the power supply voltage and a gate connected to the gate of the third transistor,   wherein the third, fifth, and sixth transistors provide the first time constant.   
     
     
       4. The driver of claim 3, wherein the output current means further comprises the third and fourth transistors. 
     
     
       5. The driver of claim 3, wherein the input current means further comprises: a seventh transistor having a source connected to the power supply voltage, a drain connected to the gate of the third transistor, and a gate connected to the first control signal, wherein the third, sixth, and seventh transistors provide the second time constant.   
     
     
       6. The driver of claim 5, wherein the input current means further comprises: an eighth transistor having a source connected to the bias curent input, a drain connected to the gate of the third transistor, and a gate connected to a second control signal from the control means, wherein the third, fifth, sixth and eighth transistors provide the third time constant.   
     
     
       7. A differential current mode driver having a plurality of modes of operation, comprising: an input current mirror receiving a biasing current and generating a reference current;   a low pass filter having a plurality of characteristic time constants in a voltage loop of the input current mirror;   a control circuit switching the low pass filter between time constants in response to a change in a present mode of operation of the differential current mode driver;   an output current mirror generating a source current and a sink current proportional to the reference current, wherein the source current and the sink current are substantially equal in magnitude and opposite in direction; and   output switches directing the source current to a first output and the sink current to a second output in response to a first state of the single-ended input signal and directing the source current to the second output and the sink current to the first output in response to a second state of the single-ended input signal.   
     
     
       8. The differential current mode driver of claim 7, wherein the plurality of modes of operation comprise a normal mode, a power-down mode, and a power-up mode. 
     
     
       9. The differential current mode driver of claim 8, wherein the control circuit switches the low pass filter to a first time constant when the differential current mode driver is in the normal mode, to a second time constant when the differential current mode driver is in the power-down mode, and to a third time constant when the differential current mode driver is in the power-up mode. 
     
     
       10. The differential current mode driver of claim 9, wherein the first time constant is longer than the second time constant and the second time constant is longer than the third time constant. 
     
     
       11. The differential current mode driver of claim 9, wherein the low pass filter comprises first, second, and third resistive elements and capacitive elements, the first resistive element and the capacitive elements providing the first time constant, the second resistive element and the capacitive elements providing the second time constant, and the first and third resistive elements and the capacitive elements providing the third second time constant. 
     
     
       12. The differential current mode driver of claim 7, wherein a portion of the output current mirror comprises a portion of the input current mirror which generates the reference current. 
     
     
       13. A method of controlling a differential current mode driver having a plurality of modes of operation, comprising: utilizing an input current mirror to provide a reference current;   generating a source current and a sink current proportional to the reference current, wherein the source current and the sink current are substantially equal in magnitude and opposite in direction; and   connecting one set of elements within a plurality of sets of elements to a control node in the current mirror to generate a time constant for a voltage at the control node, wherein each set of elements of the plurality of sets of elements corresponds to one mode of operation within the plurality of modes of operation and generates a time constant which differs from a time constant generated by other sets of elements within the plurality of sets of elements.   
     
     
       14. The method of controlling a differential current mode driver of claim 13, wherein said step of connecting one set of elements within a plurality of sets of elements to a control node further comprises: connecting a first set of elements to the control node when the differential current mode driver is in a corresponding first mode of operation to generate a first time constant;   connecting a second set of elements to the control node when the differential current mode driver is in a corresponding second mode of operation to generate a second time constant; and   connecting a third set of elements to the control node when the differential current mode driver is in a corresponding third mode of operation to generate a third time constant.   
     
     
       15. The method of controlling a differential current mode driver of claim 13, further comprising: detecting a mode of operation of the differential current mode driver utilizing a voltage at the control node.